Το απόθεμα στο κατάστημα τελειώνει, γι’ αυτό πρέπει να αγοράσουμε σήμερα.

Στο is a contraction of σε + το, so it literally means in the or at the.
Κατάστημα means store or shop.

So στο κατάστημα means in the store or at the shop.

After σε (and therefore after στο), Greek uses the accusative. So κατάστημα here is accusative singular.

Because κατάστημα is a neuter noun, its nominative and accusative singular forms are the same, so you do not see a change in the ending.

Τελειώνει is from τελειώνω, which can mean finish, end, or run out, depending on context.

Here, with το απόθεμα, the most natural meaning is is running out. So the idea is not just it ends, but more specifically the stock is getting used up.

The present tense here shows an ongoing situation: the stock is currently being used up. Greek often uses the present in places where English would say is running out.

So Το απόθεμα... τελειώνει gives the sense that this is happening now, not that it already happened.

Γι’ αυτό means for that, because of that, or more naturally in English, so, therefore, or that’s why.

It is a shortened form of για αυτό. The apostrophe shows that a vowel has been dropped in pronunciation and writing. This shortening is very common in everyday Greek.

The comma separates two linked ideas:

• Το απόθεμα στο κατάστημα τελειώνει
• γι’ αυτό πρέπει να αγοράσουμε σήμερα

The first part gives the reason, and the second part gives the result. In English, this works like The stock is running out, so we need to buy today.

Πρέπει is an impersonal verb meaning it is necessary, one must, must, or should, depending on context.

So πρέπει να αγοράσουμε literally means it is necessary that we buy, but in natural English that becomes we must buy or we should buy.

After πρέπει, Greek normally uses να plus a verb.
So:

• πρέπει να αγοράσουμε = we must buy

You generally cannot just say πρέπει αγοράσουμε. The να is part of the normal structure.

Αγοράσουμε is the aorist subjunctive form, used after να here. It refers to a single complete action: to buy.

That fits this sentence, because the idea is we need to make the purchase today.

If you said πρέπει να αγοράζουμε, that would suggest a repeated or habitual action, more like we should be buying or we should buy regularly, which is not the meaning here.

Greek word order is more flexible than English word order. Σήμερα means today, and putting it at the end is perfectly natural.

It can also add a little emphasis, as if saying we need to buy today.
You could also say Σήμερα πρέπει να αγοράσουμε, but the original version sounds very natural too.